Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen, with a light ends byproduct and a C.sub.2 -C.sub.4 recycle product. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. C., using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function. One specific example of a catalyst disclosed in U.S. Pat. No. 4,746,763 consists of a ZSM-5 type zeolite, gallium and a phosphorus containing alumina as a binder.
The conditions used for the dehydrocyclodimerization reaction result in catalyst deactivation which is believed to be caused by excessive carbon formation (coking) on the catalyst surface. After several days (from about 3 to 10 depending on the operating temperature) enough activity has been lost due to coke deposition necessitating regeneration of the catalyst. Regeneration involves burning or oxidizing the coke present on the catalyst at elevated temperatures. In addition to loss of activity due to coke formation, applicants have observed that catalysts containing a phosphorus modified alumina as a binder are gradually deactivated (over a period of time from several months to about a year) by exposure to hydrogen at temperatures greater than 500.degree. C. Applicants have found that the loss of activity due to hydrogen exposure cannot be restored by regeneration means, i.e., burning or oxidation at elevated temperatures. Therefore, there is a need for reactivating catalysts that have lost activity due to exposure to hydrogen above 500.degree. C.
Applicants have met this need by providing a reactivation process in which the catalyst is treated with aqueous solution of a weakly acidic ammonium salt or a dilute acid solution at a temperature and for a time sufficient to remove any deleterious species that have formed on the catalyst. This treatment removes some aluminum and phosphorus (and small amounts of silicon) from the catalyst as evidenced by analysis of the wash water. Next, the catalyst is oxidized in air, i.e., calcined, at a temperature and for a time sufficient to fully reactivate the catalyst.
As used in this application, regeneration refers to the process of restoring lost activity due to coke formation, while reactivation refers to the process of restoring lost activity due to hydrogen exposure.